Generally, when a newborn (child) is placed in a child car seat, a back portion of the child car seat is inclined as much as possible so that the newborn can lie on its back in the seat, because the neck of the newborn is not strong yet. When a child weighing less than 10 kg is positioned in the seat, the child car seat is fixed backward with respect to a vehicle travelling direction (seat). When an impact load acts on the front part of a vehicle with the seat fixed in the way mentioned above, the newborn's shoulder is subjected to a concentrated load through a shoulder belt.
FIG. 9 illustrates the magnitude of impact of a collision or the like acting on the newborn.
In FIG. 9, a seat belt S is applied to the newborn K who is laid at an angle of α with respect to a vertical line. Then, referring to impact force on the entire body of the newborn K caused by the collision or an emergency braking of the vehicle as F, the impact force on the shoulder of the newborn K as F1, and the impact force on the back of the newborn K as F2, the impact force F acting on the entire body of the newborn in the travelling direction of the vehicle is determined by a product of a mass of the newborn and deceleration. However, since the newborn K is braced in the child car seat, the impact force can be divided into a force component parallel to the back portion of the child car seat (namely the impact force F1 acting on the shoulder) and a force component perpendicular to the back portion of the child car seat (namely the impact force F2 acting on the back) as shown in formula (1).F=√{square root over (F1′2+F2′2)}  (1)
And since an inclination angle of the child car seat is α, the following formulas (2), (3) are obtained.F1=F sin α  (2)F2=F cos α  (3)
With the newborn K laid in the child car seat in the most inclined position, when the angle α increases, the force F1, acting on the shoulder becomes large in proportion to sin α. Moreover, while the impact force F2 acting on the back is absorbed by the entire back portion, the impact force F1 acting on the shoulder is concentrated on a contact point between the seat belt and the shoulder. Therefore, the damage to the shoulder portion can be significant.
Since the conventional child car seat fixed backward to the vehicle's travelling direction is so constructed that the impact force acts only on the shoulder belts at an impact time, the impact force concentrated on the frail shoulders of the newborn or child (hereafter referred to as “child”) caused significant damage to the shoulder.